1. Field of the Invention
The present invention relates generally to a projection optical system for use in a projection type image display apparatus such as a projector, and more particularly, to a projection optical system for projecting an image onto a projection surface in a direction which is oblique to the projection surface.
2. Description of Related Art
Conventionally, projectors or projection type image display apparatuses have been proposed in which an image forming element is illuminated with light from a light source and the modulated light which is transmitted through the image forming element or reflected by the image forming element is enlargingly projected onto a screen by a projection lens to display an image.
Some of projection optical systems for use in such projectors allow projection in a direction which is oblique to the screen in order to reduce the distance between the screen and the projector.
For example, Japanese Patent Application Laid-Open No. H05(1993)-100312 has proposed a projection optical system which enlargingly projects an image produced on the basis of an original image formed on an image forming element onto a screen. In the projection optical system, a wide-angle lens with a wide field angle is used, and the image forming element and the screen are disposed with a displacement from the optical axis of the projection optical system to use marginal portions of the field angle for the projection, thereby projecting the image in a direction oblique to the screen.
Japanese Patent Application Laid-Open No. H05(1993)-080418 has proposed a projection optical system which forms an intermediate image of luminous flux from an image forming element by a first projection optical system and enlargingly projects the intermediate image onto a screen by a second projection optical system. In the projection optical system, the optical axes of the first and second projection optical systems are appropriately inclined to project the image onto the screen in a direction oblique to the screen.
Domestic re-publication of PCT international publication for patent application WO97/01787 (corresponding to U.S. Pat. No. 5,871,266) has disclosed a projection optical system which realizes oblique projection by using a plurality of reflective surfaces.
Japanese Patent No. 2786796 has disclosed a projection optical system in which light incident on an ellipsoidal mirror at a first focal point is guided to a second focal point which is disposed to match a focal point of a parabolic mirror, and the light guided to the focal point of the inclined parabolic mirror is projected as collimated light onto a screen in a direction oblique to the screen.
Japanese Patent Application Laid-Open No. 2001-264627 has proposed an oblique projection optical system which includes an image forming optical system 602 with a positive optical power and a curved mirror 603 with a negative optical power and provides a positive optical power in total to project an image onto a screen 605, as shown in FIG. 11.
In the projection optical system proposed in Japanese Patent Application Laid-Open No. H05(1993)-100312, since the image forming element and the screen are displaced from the optical axis, a smaller amount of luminous flux passes through an entrance pupil in a portion of the field angle off the optical axis to cause uneven brightness in the projected image. In addition, the displacement of the image forming element from the optical axis leads to an increase in the size of the projection optical system.
In the projection optical system proposed in Japanese Patent Application Laid-Open No. H05(1993)-080418, the lens systems are only tilted. Thus, an increased number of lenses are needed to eliminate an astigmatic difference and sufficiently incline an image surface.
The projection optical system proposed in Domestic re-publication of PCT international publication for patent application WO97/01787 uses one concave mirror and one or two convex mirrors to form a coaxial optical system and projects an image in an oblique direction by partially using reflective surfaces of the concave mirror and the convex mirror(s). When the coaxial optical system is used, however, it is difficult to correct aberration and provide a brighter reflective optical system, that is, have a smaller F number.
In the projection optical system disclosed in Japanese Patent No. 2786796, light rays other than the principal ray do not pass through the first focal point of the ellipsoidal mirror to produce aberration, and the same light rays again cause aberration since the rays do not pass through the second focal point of the ellipsoidal mirror. The image of the light is enlarged at the time of the incidence on the parabolic mirror, so that greater aberration is produced on the screen to significantly degrade performance of image formation.
The projection optical system proposed in Japanese Patent Application Laid-Open No. 2001-264627 does not form an intermediate image since the overall system has the positive optical power. Thus, eliminating distortion of an image without affecting performance of image formation requires the curved mirror with the negative optical power disposed close to the screen, but the curved mirror has a large size.